This invention relates to thin ferroelectric films that are applicable to the manufacture of electrooptic devices, nonvolatile memory devices and the like since their surfaces are optically smooth and transparent and are free from pinholes. The invention also relates to a process for producing such thin ferroelectric films by making use of the hydrolysis of organometallic compounds.
Because of the many properties exhibited by ferroelectrics such as ferroelectricity, piezoelectricity and electrooptic effect, thin ferroelectric films have many potential applications not only in nonvolatile memories but also in surface elastic wave devices, infrared-pyroelectric devices, acousto-optic devices, electro-optic devices, etc. In particular, the application of thin ferroelectric films to electrooptic devices such as second harmonic generation (SHG) devices and optical modulating devices that have a thin-film waveguide structure has as an essential requirement the preparation of single-crystal thin films in order to achieve lower optical loss and characteristics of the single-crystal grade. In accordance with the conventionally known methods of preparing single-crystal thin films, thin epitaxial films of ferroelectrics such as BaTiO.sub.3, PbTiO.sub.3, Pb.sub.1-x La.sub.x (Zr.sub.1-y Ti.sub.y).sub.1-x/4 O.sub.3 (PLZT), LiNbO.sub.3, KNbO.sub.3 and Bi.sub.4 Ti.sub.3 O.sub.12 are formed on single-crystal oxide substrates by deposition techniques including high-frequency magnetron sputtering, ion-beam sputtering, pulsed laser deposition and MOCVD. All of these techniques require very expensive equipment for implementation. In addition, they are unsatisfactory in terms of compositional control and the surface properties of thin films; furthermore, the crystal growth temperature must be comparatively high (.gtoreq.500.degree. C.).
Examined Published Japanese Patent Application (kokoku) No. 27482/1987 teaches a process for producing thin ferroelectric films using organometallic compounds and it offers the following advantages: it achieves precise control over chemical composition; it insures uniformity at the molecular level; it can be implemented at lower temperatures; it can produce films of large area; and it can be implemented at low equipment cost.
In K. Nashimoto and M. J. Cima, "Epitaxial LiNbO.sub.3 Thin Films Prepared by Sol-Gel Process" in Materials Letters, 10, 7, 8 (1991), 348, the present inventors reported that using yet to be hydrolyzed organometallic compounds, thin films of single-crystal ferroelectrics were grown epitaxially on single-crystal substrates whereas the polycrystalline growth of thin ferroelectric films occurred when using hydrolyzed organometallic compounds or when the substrate used had no epitaxial property with respect to the thin ferroelectric film to be grown.
The problem with the method described in Examined Published Japanese Patent Application (kokoku) No. 27482/1987, supra is that only thin polycrystalline films of low density could be produced even when firing was conducted at elevated temperatures. FIG. 2 is a simplified cross-sectional view showing diagrammatically a thin polycrystalline film 4 of low density that was formed on a substrate 1 by firing at elevated temperatures. Because of this low-density defect, the polarization-derived characteristics of the ferroelectric material cannot be utilized to the fullest extent; the thin film of interest causes so much scattering of light at crystal grain boundaries or by pinholes that it is not suitable for use as an optical waveguide or the like; the film is also unsuitable for use as a capacitor on account of great current leakage and small dielectric breakdown voltage due to crystal grain boundaries or-pinholes.
The approach of preparing the epitaxial LiNbO.sub.3 thin film by the process described in Materials Letters, ibid. later turned out that when using a substrate having no epitaxial property with respect to the thin film to be formed, the temperature for the crystallization of the thin film of a hydrolyzed organometallic compound was lower than in the case where the thin film was prepared from a yet to be hydrolyzed organometallic compound.
The single-crystal or polycrystalline thin ferroelectric film that was formed by firing at a temperature of about 400.degree. C. had an optically smooth and transparent surface and the grain size of crystals was much smaller than the wavelength of light; however, due to the presence of fine pores with sizes on the order of nanometers, the thin film did not have a satisfactorily high density, nor did it have a refractive index of the single-crystal grade. The thin ferroelectric film that was formed by firing at a temperature of about 700.degree. C. had high density and its refractive index was of the single-crystal grade; however, due to the growth of secondary crystal grains, the surface of the film was not optically smooth; in addition, the film had only low transparency since the grain size of crystals was close to the wavelength of light.